The present invention is directed to methods of modifying wood pulp fiber morphology to produce three-dimensional coiled fibers without the aid of a chemical cross-linker.
Wood pulp is commonly used to make paper as well as absorbent articles. When wood pulp fibers are flat, or roughly two-dimensional, the fibers lack absorbency and softness compared to wood pulp fibers that are coiled, or three-dimensional.
Never-been-dried wood pulp has many fine pores within the cell walls in a multi-lamellar fashion. The pores are commonly referred to as intra-fiber capillaries, in contrast to inter-fiber capillaries that are formed between individual fibers. The intra-fiber capillaries of a never-been-dried pulp are highly vulnerable to outside forces such as the surface tension of water, electrolytes, mechanical and thermal treatments to name a few. In particular, intra-fiber capillaries are easily collapsed during conventional thermal drying, such as during drum drying.
When the intra-fiber capillaries of a never-been-dried pulp collapse during drying, the width, or diameter, of individual fibers shrinks. As a result, the morphology of once-dried wood pulp tends to be flat and ribbon-like, and the intra-fiber capillaries practically disappear.
If a never-been-dried fiber does not shrink uniformly during drying, its fiber morphology will be quite different from the conventional ribbon-like fiber morphology. Such fibers that shrink non-uniformly are likely to be coiled or twisted. The degree of coils or twists per individual fiber depends on the number of intra-fiber capillaries within the wood pulp and the degree of non-uniform shrinkage of fiber diameters along their fiber axes, i.e., perpendicular to the fiber diameter direction.
Flash drying is a well-known thermal drying method used to dry various materials, such as wood pulps, gypsum, and native starch. In flash drying, a wet material is exposed to a very hot drying air (or gas) environment without any constraints at a very short time, for example, a few seconds. These drying conditions of a flash dryer for wood pulp fibers can cause fibers to be in a non-equilibrium state during drying so as to make the fibers shrink non-uniformly. This results in fibers having coiled structures. In addition, such a short drying time provides very little opportunity for the pores within the fibers to collapse, thereby resulting in enhanced absorptive properties for the fibers.
Unfortunately, however, flash drying conditions also have a tendency to cause fibers to be entangled, thus forming a so-called fish eye (or fiber bundles, nodules), not just causing the fibers to be twisted. Consequently, a typical commercial flash dryer has been designed to minimize the fiber entanglements and twists.
Curly, twisted cellulose fibers can be produced by permanently interlocking the intra-fiber capillaries with a chemical cross-linker prior to flash drying. The use of a chemical cross-linker is unfavorable for a number of reasons. In particular, the use of a chemical cross-linker involves safety concerns since chemical cross-linkers are generally hazardous and harmful. Therefore, the use of a chemical cross-linker requires a thorough washing of un-reacted chemical cross-linker for safety. Also, the use of a chemical cross-linker is likely to cause interlocking between fibers that would be difficult to be fiberized into individual fibers for a product application. Potential damage to the fibers may occur during the defibration stage due to interlocking of the fibers. It can be difficult to form an absorbent product due to such interlocking of fibers. Furthermore, the use of a chemical cross-linker is not very economical due to the complexity of handling such a chemical cross-linker.
More importantly, with respect to the present invention, such permanently interlocking intra-fiber capillary structures tend to make the fibers stiffened and destroy all the useful capillaries as fluid channels.
In order to obtain a very short drying time during a thermal drying process such as flash drying, it is necessary to fluff pulp so that the largest possible pulp surface is exposed to the hot drying air. In particular, the wet pulp must be thoroughly fluffed as individual fibers prior to flash drying, otherwise dried fibers come out as fiber bundles, or entanglements. Once fibers are entangled during flash drying, the fibers are very difficult to be disentangled into individual fibers for subsequent uses. Thus, the fluffing operation is the key to a successful flash drying system.
To use a feed of wood pulp and/or various other hydrophilic materials for flash drying, the water in the wet pulp should be removed substantially. This water removal is conventionally achieved by mechanical means such as a filter press or centrifugation. At such a high consistency level, it is very difficult to fluff the pulp into individual fibers. To alleviate this problem, a mechanical device such as a disintegrator is commonly employed after the mechanical de-watering step in the flash drying system.
It is therefore an object of the present invention to provide a method of modifying wood pulp fiber morphology to produce three-dimensional coiled fibers without the aid of a chemical cross-linker.
It is another object of the present invention to maintain the original intra-capillary structure of wood pulp fiber during a rapid thermal drying process to utilize the structure of the wood pulp fiber as fluid channels.
It is yet another object of the present invention to use a drying aid, such as a surfactant, to maximize the extent of twisting and to help maintain the porous structures of wood pulp fibers during a rapid thermal drying process.
It is still another object of the present invention to provide an alternative method of fluffing wet pulp into individual fibers as opposed to using conventional mechanical devices.